Such a device is known from German Patent No 647 287, which describes a windmill having a hinge connection between a rotor blade and the rotary support with a hinge axis extending at rights angles both to the longitudinal axis of the rotor blade and to the axis of rotation of the support. By pivoting the rotor blades or parts thereof from a position in which they are essentially at right angles to the wind direction to an orientation in which they are at a small angle or parallel to the wind direction, the wind-braking area of the rotor can be adjusted to the actual wind speed.